Currently, soft contact lenses are widely used for correcting many different types of vision deficiencies. Such wide adoption of soft contact lenses is partly due to their relatively-low elastic modulus. Unlike hard contact lenses (e.g., RGP lenses), soft contact lenses can be worn for sufficiently long periods of time and can provide patients with the benefits including adequate initial comfort (i.e., immediately after lens insertion), relatively short period of adapting time required for a patient to become accustomed to them, and/or proper fit. However, because the cornea does not receive oxygen from the blood supply like other tissue and because soft contact lenses conform closely to the shape of the eye so that oxygen cannot easily circumvent the lens, soft contact lenses must allow oxygen to diffuse through the lens to reach the cornea, namely having a relatively high oxygen transmissibility (i.e., oxygen permeability over the lens thickness) from the outer surface to the inner surface to allow sufficient oxygen permeate through the lens to the cornea and to have minimal adverse effects on corneal health. If sufficient oxygen does not reach the cornea, corneal swelling occurs. Extended periods of oxygen deprivation cause the undesirable growth of blood vessels in the cornea. Oxygen permeability and elastic modulus of silicone hydrogel contact lenses play a critical role in lens comfort and corneal health.
High oxygen permeable silicone hydrogel materials have been developed to have a high oxygen permeability (Dk) and to make contact lenses capable of providing corneal health benefits. But, wearing comfort requirement necessitates a relatively low modulus. The ability to decrease elastic modulus of a silicone hydrogel without negatively impacting oxygen permeability has been a challenge in the contact lens industry. Several approaches have been developed, including, for example, increasing the percentage hydrophilic monomer concentration in a lens formulation, lowering the percentage of silicon-containing macromer (e.g., betacon or polydimethylsiloxane (PDMS) macromer) in a lens formulation, using a customized modulus-lowering monomer in a polymer formulation (U.S. Pat. No. 5,908,906), using a combination of mono- and di-vinyl functionalized macromers, or combinations thereof. Each of the known approaches have one or more of the following disadvantages. First, lowering modulus is often accompanied by decreasing Dk. Second, use of customized modulus-lowering monomer(s) or mono-vinyl-functionalized macromer(s) can increase lens production cost. Third, they may cause undesirable changes in physical properties of a silicone hydrogel material. For example, replacement of methacrylate by a corresponding acrylate in a lens formulation can lower not only the modulus but also the glass transition and hardness of a resultant silicon hydrogel material and therefore adversely affect the lathing ability of the resultant hydrogel material. Because of one or more above-described disadvantages, the known approaches have limitations in their practical use in the contact lens manufacturing.
It would be desirable to have a silicone hydrogel material that has high oxygen permeability and low modulus (e.g. preferably less than 1.0 MPa). Therefore, there are needs for a method of increasing oxygen permeability (Dk) and lowering or at least maintaining the elastic modulus of silicone hydrogel lenses, for a formulation capable of forming a silicone hydrogel material having relatively high oxygen permeability and low modulus, and for silicone hydrogel contact lenses having relatively high oxygen permeability and low modulus.